The present invention relates to polyester resins, such as polyethylene terephthalate, which are widely used for various types of packaging containers, films, fibers or the like. More particularly, it relates to polyester resins produced by using inexpensive and environmentally safe titanium as a component of the polycondensation catalyst, said resins having an excellent color tone and melting heat stability, forming few by-products in their production, excellent transparency and especially suited for molding bottles and the like.
Polyester resins such as polyethylene terephthalate are excellent in many properties such as mechanical strength, chemical stability, gas barrier properties and hygienic qualities and are also light in weight, so that they are popularly used for various kinds of packaging containers, films, fibers, etc.
Such polyester resins have been produced mostly by using an antimony compound as polycondensation catalyst.
However, the polyester resins polymerized by using an antimony compound as polycondensation catalyst, when for instance molded into a container for a drink or food and put to practical use as such, have a probability that antimony remaining in the resin might be eluted out from the container when placed under a high temperature and transfer, if small in quantity, into the drink or food in the container. Use of antimony, which is a heavy metal, also poses the problem of environmental pollution. Thus, use of very scanty amount of antimony compound even though using thereof for the production of polyester resins or preferably use of a more safe metal as catalyst (antimony-free catalyst) for the production of polyester resins has been desired. The obtained molded containers cannot necessarily show a sufficient transparency and, therefore, frequently failed to meet the requirements of the market.
Polyester resins polymerized by using a germanium compound instead of an antimony-containing catalyst are also known. Germanium compounds, however, are very expensive because of scanty reserves, so that the development of a catalyst that can substitute for such costly compounds has been required. When such a polyester is blow-molded, a blowing mold used therefor tends to be contaminated, so that the obtained molded containers are deteriorated in surface smoothness and transparency. In addition, there also arises such a problem that the yield of these containers is considerably deteriorated due to necessity of additional steps for cleaning the contaminated mold.
On the other hand, it is considered that the mold contamination is caused by the transfer of by-products such as cyclotrimethylene terephthalate (cyclic trimer) which are produced upon forming a preform by injection-molding the resin and contained in the molded preform, onto the surface of the blowing mold when subjecting the cyclic trimer or the like to stretch blow molding. Consequently, for the purpose of inhibiting the generation of such by-products upon forming the preform by injection-molding method, for example in Japanese Patent Publication (KOKOKU) No. 7-37515(1995), there has been proposed the method of deactivating a catalyst contained in the resin by contacting the polycondensed resin with hot water heated to a temperature of 50 to 100xc2x0 C. However, as a result of the present inventors"" studies, it has been found that the step of contacting the polycondensed resin with hot water is required thereby increasing the production cost of polyester, and molded products obtained by the above conventional method exhibit a considerably poor transparency. Further, it is suggested that the poor transparency of the molded products is caused by not the above-described mold contamination but deterioration in quality of the resin itself.
Various types of polyester resins polymerized by using an inexpensive and safe titanium compound have also been proposed. The titanium catalyst can be used in a small amount as compared to antimony and germanium catalysts due to its high catalytic activity, and is free from problems concerning safety unlike the antimony catalyst, and inexpensive as compared to the germanium catalyst. Therefore, it is considered industrially valuable to study and develop a polyester resin produced by processes using these titanium catalysts. However, polyester resins obtained by the polymerization using such titanium catalysts exhibit a yellowish color, and are deteriorated in thermal stability since these resins undergoes a significant discoloration after heating.
In Japanese Patent Application Laid-Open (KOKAI) No. 8-73581 (EP-A-0699700), an antimony-free, colorless high-transparency polyester produced by using specified amounts of a complex forming agent and a titanium compound has been proposed. According to the follow-up tests conducted by the present inventors, however, it was found that the polyester resin obtained by the method described in the above patent contained large quantities of such by-products as acetaldehyde and cyclic trimers, and had the problem that, when molded into a food container such as a bottle, the obtained bottle might affect the savor of the content, or the mold used for bottle molding could be excessively stained.
Further, in the process according to the above reference, cobalt which is a heavy metal and used as bluish agent is used in order to improve the color tone. However, in view of environmentally safe, it is desired that cobalt which is a heavy metal is not used.
As a result of the present inventors"" earnest studies to solve the above problem, it has been found that by using environmentally safe and inexpensive titanium as a polycondensation catalyst constituent and by selecting the optimal catalyst components and amount thereof and polycondensation conditions, the above problem can be solved.
The present invention has been attained on the basis of the above finding.
The object of the present invention is to provide a polyester resin in which titanium with high environmental safety is used as a component of the polycondensation catalyst, the resin shows excellent color tone and melting heat stability and forms few by-products such as acetaldehyde and cyclic trimers in the course of its production, and that therefore when the resin is molded into a food container such as a bottle, the produced bottle won""t affect the savor of its content, and the mold used for bottle molding is scarcely stained.
To attain the above aim, in the first aspect of the present invention, there is provided a polyester resin produced by polymerizing a dicarboxylic acid component comprising mainly an aromatic dicarboxylic acid or its ester forming derivative, and a diol component comprising mainly ethylene glycol in the presence of a catalyst containing a titanium compound, which polyester resin contains titanium atoms (Ti) in an amount of 0.002 to 1.0 mole based on one ton of the polyester resin and has the following properties:
Intrinsic viscosity ([xcex7], dl/g)xe2x89xa70.70
Hunter""s xe2x80x9cbxe2x80x9d valuexe2x89xa64
Acetaldehyde content (AA0, ppm)xe2x89xa65.0
In the second aspect of the present invention, there is provided a polyester hollow container molded by using a polyester resin according to the first aspect.
In the third aspect of the present invention, there is provided a process for producing a polyester resin which comprises polymerizing a dicarboxylic acid component comprising mainly an aromatic dicarboxylic acid and a diol component comprising mainly ethylene glycol in the presence of a catalyst containing (1) a titanium compound, (2) a phosphorus compound and (3) at least one compound selected from compounds of Ia-Group metals except for hydrogen, IIa-Group metal compounds and manganese compounds, said compounds (1) to (3) being added to the reaction system in the order of (2), (3) and (1).